1. Field of the Invention
The present invention relates to technical fields of an information recording medium, an information recording apparatus, an information recording method, an information playback apparatus, an information playback method, a recording program recording medium, and a playback program recording medium. More particularly, the invention relates to technical fields of an information recording medium to/from which information is optically recorded/played back, an information recording apparatus, an information recording method, and a recording program recording medium for optically recording information to the information recording medium, and an information playback apparatus, an information playback method, and a playback program recording medium for optically playing back information recorded on the information recording medium.
2. Description of Related Art
In recent years, an optical disk of high recording density such as so-called DVD (Digital Versatile Disc) is becoming common. Concretely, not only a playback-only DVD but also a recordable DVD are becoming common.
On the other hand, recently, an optical disk which has realized higher recording density by using a so-called blue laser is being practically used. Improvement in recording density of such an optical disk is increasing demand.
The spatial frequency is widely used as a scale indicative of recording density of an optical disk. The spatial frequency is defined as “an amount expressing fineness of an image of a series of lines at equal intervals or sine waves or a cyclic structure of an object” and is a physical amount expressed by the number of cycles per unit length. In the case of applying the spatial frequency to an optical disk, the spatial frequency is a parameter indicative of the size (fineness) of a pit formed in the optical disk.
In the case of optical information recording/playback using an optical disk, the spatial frequency is determined by the wavelength of a light beam for reproduction with which the optical disk is irradiated, and the numerical aperture of an objective lens for focussing the light beam onto the optical disk.
On the other hand, as a technique of effectively increasing the spatial frequency determined by the wavelength of the light beam and the numerical aperture of an objective lens more than a value determined by the actual wavelength and the numerical aperture, techniques of so-called super-resolution are known. Among them, a fluorescent super resolution technique using a phosphor as a recording layer on an optical disk is known (refer to, for example, from line 1 of the upper right column to line 14 of the lower right column of page 3 in Japanese Patent Application Laid-Open (JP-A) No. H02-050328). The disclosures of the corresponding U.S. Pat. Nos. 4,927,681 and 5,063,556 are incorporated by reference in their entirety. A fluorescent super-resolution image is formed by forming the pit on an optical disk by using a phosphor which emits light when being irradiated with a playback light beam in an attempt to increase the effective spatial frequency at the time of irradiation of the light beam by about twice.
On the other hand, as another method of increasing the spatial frequency, a method of effectively decreasing the size of an irradiation area of a light beam (light spot) focused on a recordable optical disk at the time of forming a pit by irradiating the optical disk with a recording light beam to thereby reduce the physical size itself of the pit has been also developed. One method includes a super-resolution technique using a photochromic layer as a cover layer (refer to, for example, JP-A No. H07-072567 (for example, paragraphs “0010” to “0020” and FIG. 5)). In the super-resolution technique using the photochromic layer, the photochromic layer functions as a small aperture to a playback light beam, the diameter of a light spot formed on an optical disk by the playback light beam becomes equivalently smaller and, as a result, information recording/playback of high recording density can be achieved.
In the above-mentioned conventional super-resolution technique using the photochromic layer, however, the photochromic layer itself does not have the function of carrying information, so that a recording layer for carrying information need to be provided so as to be stacked on the photochromic layer. As a result, a pit corresponding to information to be recorded need to be formed in the recording layer by using a recording light beam which passes through the photochromic layer and reaches the recording layer at the time of recording information.
In contrast, the photochromic layer in the super-resolution technique has the functions of changing energy density distribution of a recording light beam, and the light transmitting characteristic of increasing the transmission rate and saturating the transmission rate, thereby equivalently reducing the size of a light spot. When a light beam for recording passes through the photochromic layer itself, a so-called light absorbing phenomenon occurs. The intensity of a light beam for recording reaching the recording layer therefore decreases. Consequently, to complete recording of information, a light beam for recording having a high recording power need to be used.
The above problem causes complication of the configuration of the information recording apparatus, and increase in the manufacturing cost.
Also, at the time of playing back information recorded in the recording layer, the light transmission characteristic of the photochromic layer is changed by irradiation of the light beam for playback, and the recorded information to be played back with deterioration.
Further, in the case of playing back information recorded in the recording layer by detecting, for example, a change in the polarization angle of a playback light beam emitted to the recording layer, the photochromic layer functions as a small aperture. In this case as well, however, a playback light beam having proper intensity, which does not destroy a pit for carrying information formed in the recording layer need to be used. As a result, stability and reliability at the time of playing back information become insufficient.
The invention has been achieved in consideration of the above problem, and an object of the invention is, for example, to provide an information recording medium capable of recording information at recording density higher than that in the conventional technique, and improving stability and reliability at the time of playing back recorded information, and an information recording apparatus, an information recording method, and a recording program recording medium for optically recording information onto the information recording medium, and an information playback apparatus, an information playback method, and a playback program recording medium for optically playing back information recorded on the information recording medium.